Bragg grating erosion sensor for harsh environment

Abstract

An erosion sensor includes one or more optical fibres each having one or more cores and an optical sheath surrounding the one or more cores, a variable pitch measurement Bragg grating inscribed in one of the cores of one of the optical fibres over a measurement section to be eroded, and one or more reference Bragg gratings each inscribed in one of the cores. The reference Bragg grating is used to correct the physical length of the measurement section determined from the width of the spectrum reflected by the measurement Bragg grating, taking into account the thermomechanical parameters imposed on the erosion sensor.

1 . A Bragg grating erosion sensor comprising:
 one or more optical fibres, each optical fibre comprising one or more cores and an optical sheath surrounding the one or more cores,   a variable pitch measurement Bragg grating inscribed in one of the cores of one of the optical fibres over a measurement section to be eroded, and   one or more reference Bragg gratings, each reference Bragg grating being inscribed in one of the cores of one of the optical fibres.   
     
     
         2 . The erosion sensor according to  claim 1 , wherein at least one of the one or more reference Bragg gratings, is a constant pitch Bragg grating. 
     
     
         3 . The erosion sensor according to  claim 1 , wherein the measurement Bragg grating and at least one of the one or more reference Bragg gratings are inscribed in a same core of an optical fibre. 
     
     
         4 . The erosion sensor according to  claim 1 , wherein at least one Bragg grating is formed by a set of bubbles. 
     
     
         5 . The erosion sensor according to  claim 1 , wherein one of the one or more optical fibres comprises a measurement core and a reference core, the measurement Bragg grating being inscribed in the measurement core and at least one of the one or more reference Bragg gratings being inscribed in the reference core. 
     
     
         6 . The erosion sensor according to  claim 5 , wherein each reference Bragg grating inscribed in the reference core is arranged to reflect radiation in a wavelength range comprised in a range of wavelengths reflected by the measurement Bragg grating. 
     
     
         7 . The erosion sensor according to  claim 1 , wherein the measurement section extends between a proximal end, arranged to receive incident radiation, and a distal end, arranged to be eroded, the one or more reference Bragg gratings comprising a first reference Bragg grating inscribed on a first reference section located in a vicinity of the proximal end. 
     
     
         8 . The erosion sensor according to  claim 7 , wherein the first reference section is located upstream of the measurement section. 
     
     
         9 . The erosion sensor according to  claim 7 , wherein the first reference section is included in the measurement section. 
     
     
         10 . The erosion sensor according to  claim 1 , wherein the one or more reference Bragg gratings comprise a plurality of second reference Bragg gratings distributed along the measurement section. 
     
     
         11 . The erosion sensor according to  claim 10 , wherein one of the one or more optical fibres comprises a measurement core and a reference core, the measurement Bragg grating being inscribed in the measurement core and the second reference Bragg gratings being inscribed in the reference core, each second reference Bragg grating having a pitch equal to a local pitch of the measurement Bragg grating. 
     
     
         12 . The erosion sensor according to  claim 11  further comprising a processing unit configured to measure a spectral width of radiation reflected by the measurement Bragg grating, to measure a wavelength of radiation reflected by at least one reference Bragg grating, and determine a physical length of the measurement section according to the spectral width of the radiation reflected by the measurement Bragg grating and the wavelength of the radiation reflected by the at least one reference Bragg grating. 
     
     
         13 . The erosion sensor according to  claim 12 , wherein the processing unit is configured to determine the physical length of the measurement section according to the spectral width of the radiation reflected by the measurement Bragg grating and to a wavelength difference between a wavelength of the radiation reflected by the measurement Bragg grating and the wavelength of the radiation reflected by the at least one reference Bragg grating. 
     
     
         14 . The erosion sensor according to  claim 12 , wherein the processing unit is configured to measure a wavelength of the radiation reflected by each of the second reference Bragg gratings, to determine a variation of a physical parameter along the measurement section according to the wavelengths of the radiation reflected by the second reference Bragg gratings, and to determine the physical length of the measurement section according to the spectral width of the radiation reflected by the measurement Bragg grating and the variation of the physical parameter along the measurement section. 
     
     
         15 . A method for determining a physical length of the measurement section of the erosion sensor according to  claim 1 , comprising the steps of:
 measuring a spectral width of radiation reflected by the measurement Bragg grating,   measuring a wavelength of radiation reflected by at least one of the one of more reference Bragg gratings, and   determining the physical length of the measurement section according to the spectral width of the radiation reflected by the measurement Bragg grating and the wavelength of the radiation reflected by the at least one reference Bragg grating.